1. Field of the Invention
Generally, the present invention relates to the field of fabrication of integrated circuits, and more, particularly, to semiconductor devices having metal-silicide portions on semiconductor regions to reduce the sheet resistance of the semiconductor regions, and a method of manufacturing these semiconductor devices.
2. Description of the Related Art
In modem ultra-high density integrated circuits, device features are steadily decreasing to enhance device performance and functionality. Shrinking the feature sizes, however, entails certain problems that may partially offset the advantages obtained by the reduced feature sizes. Generally, reducing the feature sizes of, for example, a transistor element, leads to a decreased channel resistance in the transistor element and thus results in a higher drive current capability and enhanced switching speed of the transistor. In decreasing the feature sizes of these transistor elements, however, the increasing electrical resistance of conductive lines and contact regions, i.e., of regions that provide electrical contact to the periphery of the transistor element, becomes a dominant issue since the cross-sectional area of these lines and regions decreases with decreasing feature sizes. The cross-sectional area, however, determines, in combination with the characteristics of the material comprising the conductive lines and contact regions, the resistance of the respective line or contact region.
The above problems may be exemplified for a typical critical feature size in this respect, also referred to as a critical dimension (CD), such as the extension of the channel of a field effect transistor that forms below a gate electrode between a source region and a drain region of the transistor. Reducing this extension of the channel, commonly referred to as channel length, may significantly improve device performance with respect to fall and rise times of the transistor element due to the smaller capacitance between the gate electrode and the channel and due to the decreased resistance of the shorter channel. Shrinking of the channel length, however, also entails the reduction in size of any conductive lines, such as the gate electrode of the field effect transistor, which is commonly formed of polysilicon, and the contact regions that allow electrical contact to the drain and source regions of the transistor, so that, consequently, the available cross-section for charge carrier transportation is reduced. As a result, the conductive lines and contact regions exhibit a higher resistance unless the reduced cross-section is compensated for by improving the electrical characteristics of the material forming the lines and contact regions, such as the gate electrode, and the drain and source contact regions.
It is thus of particular importance to improve the characteristics of conductive regions that are substantially comprised of semiconductor material such as silicon. For instance, in modem integrated circuits, the individual semiconductor devices, such as field effect transistors, capacitors, and the like, are primarily based on silicon, wherein the individual devices are connected by silicon lines and metal lines. While the resistivity of the metal lines may be improved by replacing the commonly used aluminum by, for example, copper, process engineers are confronted with a challenging task when an improvement in the electrical characteristics of silicon-containing semiconductor lines and semiconductor contact regions is required.
With reference to FIGS. 1a and 1b, an exemplary process for manufacturing an integrated circuit containing, for example, a plurality of MOS transistors, will now be described in order to illustrate the problems involved in improving the electrical characteristics of silicon-containing semiconductor regions in more detail.
In FIG. 1a, a semiconductor structure 100 includes a substrate 101, for example, a silicon substrate, in which a first semiconductor element 110 and a second semiconductor element 130 are formed. The first semiconductor element 110 may, as depicted in FIG. 1a, represent a field effect transistor of a first conductivity type, such as an n-channel transistor, and the second semiconductor element 130 may represent a field effect transistor of a second conductivity type, such as a p-channel transistor. The first semiconductor element 110 comprises shallow trench isolations (STI) 113 that are formed of an insulated material, such as silicon dioxide, and that define an active region 112 in the substrate 101. A gate electrode 115 is formed over a gate insulation layer 118 that separates the gate electrode 115 from the active region 112. Spacer elements 116 made of, for example, silicon dioxide or silicon nitride, are located at the sidewalls of the gate electrode 115. In the active region 112, source and drain regions 114 are formed and exhibit an appropriate dopant profile required to connect to a conductive channel that builds up between the drain and the source region during operation of the first semiconductor element 110.
The second semiconductor element 130 comprises substantially the same parts as the first semiconductor element 110 and corresponding parts are denoted by the same reference numerals except for a “leading 13” instead of a “leading 11.” As previously noted, the second semiconductor element 130 may differ from the first semiconductor element 110 in, for example, type of conductivity, that is, type and concentration of dopants provided in the active regions 112 and 132, lateral extension of the gate electrode, also referred to as gate length, cross-sectional area, and the like. Moreover, it should be noted that although the first and second semiconductor elements 110 and 130 in FIGS. 1a and 1b are depicted as transistor elements, the first and second semiconductor elements 110 and 130 may represent any silicon-containing region that is used for charge carrier transportation. For example, relatively long polysilicon lines may connect semiconductor elements on different locations of a single chip area and these polysilicon lines may be regarded as first and second semiconductor elements 110, 130, the electrical characteristics of which are to be improved so as to obtain an enhanced device performance with respect to signal propagation delay.
Again referring to FIG. 1a, in particular the gate length of the first and second semiconductor elements 110 and 130 determines the channel length of these devices and, therefore, as previously pointed out, significantly affects the electrical characteristics of the first and second semiconductor elements 110 and 130, whereby a reduced gate length will result in an increased resistance of the gate electrodes 115, 135 owing to the reduction of the cross-sectional area of the gate electrodes 115, 135.
A typical process flow for forming the semiconductor structure 100 may comprise the following steps. After the formation of the shallow trench isolations 113 and 133 by well-known photolithography techniques, implantation steps are performed to create a required dopant concentration in the active regions 112 and 132. Subsequently, the gate insulation layers 118 and 138 are formed according to design requirements. Thereafter, the gate electrodes 115 and 135 are formed by patterning, for instance a polysilicon layer, by means of sophisticated photolithography and trim etch methods. Then, a further implantation step for forming so-called source and drain extensions within the source and drain regions 114 and 134 is performed and the spacer elements 116 and 126 are formed by deposition and anisotropic etching techniques. The spacer elements 116 and 126 are used as an implantation mask for a subsequent implantation step in which dopant particles are implanted into the source and drain regions 114 and 134 to create the required high dopant concentrations in these regions. It is to be noted that the dopant concentration varies in FIG. 1a in the horizontal direction, i.e., in the length direction of the gate electrodes 115, 135, as well as in the vertical direction, which will hereinafter be referred to as depth direction. Although the dopant profile of the source and drain regions 114 and 134 is depicted as a region having a sharp boundary, in reality the dopant profile varies continuously due to the nature of the implantation process and the subsequent annealing steps that are performed for activating the implanted atoms and for curing the crystalline damage caused by the implantation step. Usually, the dopant profile has to be selected in conformity with other parameters of the first and second semiconductor elements 110 and 130. For example, a short gate length, and thus a short channel length, requires a “shallow” dopant profile in order to avoid the so-called “short channel effect.” Accordingly, the peak concentration in the depth direction may be located a few hundred nanometers below the surface of the drain and source regions 114 and 134. Moreover, p-channel transistors may require a different dopant profile than an n-channel transistor element.
As previously noted, the cross-section of the gate electrodes 115 and 135, which may be considered as polysilicon lines, as well as the contact area on top of the source and drain regions 114 and 134, significantly influence the electrical characteristics of the first and second semiconductor elements 110 and 130. Since, generally, these device areas primarily contain a semiconductor material such as silicon in crystalline, polycrystalline and amorphous form, these areas, although they usually include dopants, exhibit a relatively high resistance compared to, for example, a metal line. Consequently, these areas are treated to enhance the conductivity of these regions, thereby improving the overall performance of the devices.
To this end, according to FIG. 1a, a metal layer 140 is deposited over the first and second semiconductor elements 110 and 130. Typically, the metal layer 140 comprises titanium, cobalt or other refractory metals. Subsequently, a first heat treatment, for example, a rapid thermal annealing, is carried out to initiate a chemical reaction between the silicon in the source and drain regions 114, 134, the gate electrodes 115, 135 and the metal contained in the metal layer 140. If, for example, the metal layer 140 substantially comprises cobalt, an average temperature of the first heat treatment may be set to about 400° C. to create a meta-stable cobalt silicon compound exhibiting a relatively high resistivity. Since the silicon contained in the spacer elements 116, 136 and the shallow trench isolations 113, 133 is chemically bound in the form of dioxide or nitride, the metal of the metal layer 140 does not substantially react with the material of the spacer element 115, 136 and the shallow trench isolations 113, 133. After the first heat treatment, the material of the metal layer 140 that has not reacted with the underlying material is removed by, for example, a selective wet etching process. Thereafter, a second heat treatment is performed, for example, a second rapid annealing step with a temperature higher than in the first annealing step, to convert the meta-stable metal-silicon compound into a metal silicide. In the above example, when cobalt is used, a cobalt disilicide is formed in the second annealing step. The metal silicide shows a significantly lower resistance than the meta-stable metal-silicon compound, as well as a significantly lower resistance, by a factor of about 5-10, than the sheet resistance of the doped polysilicon.
FIG. 1b schematically shows the finally obtained first and second semiconductor elements 110 and 130 having formed on the respective source and drain regions 114, 134 and the gate electrodes 115, 135 a metal silicide region 141. Although the metal silicide regions 141 significantly improve the electrical characteristics of the first and second semiconductor elements 110 and 130, there is still room for improvement since, in the conventional process flow, the metal silicide regions 141 have to be formed so as to meet the requirements of the first semiconductor element 110 and the second semiconductor element 130, so that optimizing the characteristics of the silicide regions 141 of the first semiconductor element 110 compromises the effect of the suicide regions 141 of the second semiconductor element 130, and vice versa.
It is thus desirable to have a semiconductor and a method of forming the same in which the characteristics of the conductive semiconductor regions may be individually optimized for different semiconductor elements.